Brat meddles occasionally for jam (6)
I believe the answer is:
impede
'for jam' is the definition.
(synonyms)
'brat meddles occasionally' is the wordplay.
'brat' becomes 'imp' (I've seen this in another clue).
'occasionally' means one should take alternating letters.
The alternate letters of 'meddles' are 'ede'.
'imp'+'ede'='IMPEDE'
(Other definitions for impede that I've seen before include "Hinder, inhibit" , "Hinder, be obstructive" , "Delay, be an obstacle" , "Block progress of (something)" , "Delay; block progress of" .)